Make rosemary simple syrup. Place 1/2 cup of sugar and 1/2 cup of water in a pot. Bring to a boil and then remove from heat. Add rosemary sprigs. Stir and let cool. Chill until ready to use.
Core and slice the peaches
Muddle the peach slices and 1/2 the cranberries with the rosemary simple syrup and lemon juice until all of the juices have been released.
Shake with ice in a cocktail shaker.
Pour evenly between two glasses.
Top off with Prosecco and garnish with extra peach slices, cranberries and rosemary sprigs dipped with sugar.